This invention relates to fuel cells and, in particular, to end plates and heat exchangers for fuel cell systems. More specifically, this invention relates to highly integrated, compact heat exchangers for use in superheating fuel gas for high temperature fuel cells.
A fuel cell is a device which directly converts chemical energy stored in a fuel such as hydrogen or methane into electrical energy by means of an electrochemical reaction. This differs from traditional electric power generating methods which must first combust the fuel to produce heat and then convert the heat into mechanical energy and finally into electricity. The more direct conversion process employed by a fuel cell has significant advantages over traditional means in both increased efficiency and reduced pollutant emissions.
In general, a fuel cell, similar to a battery, includes a negative (anode) electrode and a positive (cathode) electrode separated by an electrolyte which serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. To achieve this, gas flow fields are provided adjacent to the anode and cathode through which fuel and oxidant gas are supplied. In order to produce a useful power level, a number of individual fuel cells must be stacked in series with an electrically conductive separator plate between each cell.
In a conventional fuel cell stack for stationary power applications, the active area of the fuel cells is large, typically between ½ and 1 m2. In order to apply a reasonable interface pressure on the cells, a large compressive load must be applied to the cells through the end plates. As the end plates must remain flat to insure intimate contact is maintained with the cells, the end plates are typically thick relative to their length and width. This thickness adds to the overall length of the fuel cell stack and size of the fuel cell power plant.
In addition, for high temperature fuel cell systems, a heat exchanger is required to heat the fuel gas to near the temperature of the stack prior to delivery to the stack. In one type of fuel cell system, this heat exchanger is placed external to the fuel cell stack as part of the balance of the plant. This requires additional space to accommodate the fairly thick insulation (2-3 inches) used to encase the heat exchanger. Also, in this type of system, process gas must be piped to and from the heat exchanger, adding to both the size and cost of the system.
As described in U.S. Pat. No. 5,856,034, insulation for the heat exchanger can be eliminated by placing the heat exchanger inside the already insulated fuel cell module enclosing the fuel cell stack. Specifically, the heat exchanger is placed upstream and adjacent the cathode inlet face of the stack, making it necessary to construct the exchanger large enough so as to completely cover the cathode inlet face. Also, in this system, due to the inherent non-uniform temperature distribution at the outlet of the heat exchanger, the stack inlet temperature distribution is also non-uniform. This condition is undesirable as non-uniform cathode inlet temperature not only creates a potential performance variation in the stack but also creates the risk of cell-to-cell wet seal leaks due to thermal expansion differences of the stack face.
U.S. Pat. No. 5,009,968 describes an end plate structure in which a thin membrane is used to maintain good electrical contact with the end cells of the fuel cell stack. The thin membrane structure is not specifically adapted to uniformly collect electrical current from the stack. U.S. Pat. No. 4,719,157 describes a thin end plate with multiple current collecting terminals used to inhibit deformation of the plate. Again, this arrangement is not specifically adapted to provide uniform collection of electrical current.